The invention relates to a matrix display, in particular an active matrix display such as a polymer light emitting diode (poly-LED) array, organic light emitting diode (OLED) array, or an active matrix liquid crystal display.
A typical prior art active matrix polymer or organic light emitting diode display has a matrix of light emitting diodes arranged as row and columns. A single pixel of such a display is illustrated in FIG. 1. The pixel 2 is connected to a row select line 4 and a data line 6. A thin film transistor 8 acts as a select transistor, to connect the electronics in the pixel 2 to the data line 6 in accordance with a signal on the select line 4. The select transistor 8 is a thin film transistor having its gate 10 acting as a control terminal connected to the select line 4. The source 12 is connected to the data line 6, and the drain 14 to the remaining components of the pixel. In particular, the drain 14 is connected to the gate or controlled terminal 18 of a pixel thin film transistor 16. The source 20 of the pixel thin film transistor 16 is connected to ground 24, and the drain 22 is connected through light emitting diode 25 to voltage rail 26. A storage capacitor 28 is connected between ground 24 and the control terminal 18 of the further thin film transistor 16.
In use, a charge is loaded through the select transistor 8 when the row select line 4 is appropriately driven. The charge is stored on capacitor 28 which controls the pixel thin film transistor 16 to be in a conducting or a non-conducting state. When the pixel transistor 16 is in a non-conducting state, i.e. switched off, no current passes through the LED and accordingly the pixel element is dark. Conversely, when the voltage on the capacitor 28 is such that the thin film transistor is switched on, current passes between voltage rail 26 and ground 24 through the light emitting diode 25 to make the pixel element bright.
It will be appreciated that in a real display there will be many rows and columns of pixels, and many corresponding select lines 4, data lines 6 and voltage rails 26.
A problem with this type of arrangement is that it is necessary to continually update the signals to continually refresh the voltage on capacitor 28. Thus, every frame every pixel needs to be addressed and the voltage across the capacitor adapted. Whereas this is necessary for displaying dynamic or rapidly changing data, the requirement to continually and repetitively supply data signals with the associated dissipation of power is not needed when a static image is being displayed.
The same considerations apply to other forms of matrix display, in particular to active matrix liquid crystal displays. Again, the liquid crystal pixel has effectively a capacitance that needs to be regularly recharged by continually and repetitively supplying data signals. Again, this wastes power.
Accordingly, it would be beneficial to provide a display that can display static images without the need for continual repetitive data supply, while still maintaining the flexibility of displaying dynamic images where required.
According to the invention, there is provided a matrix display, comprising: a plurality of data lines; a plurality of select lines; and a plurality of pixel elements including: a select transistor connected to a data line, the select transistor having a control terminal connected to a select line, a capacitance for storing charge supplied by the said data line when the pixel element is selected by the select line, the charge providing a voltage across the capacitance, and a display component connected to the capacitance for displaying a pixel image element in accordance with the voltage across the capacitance; characterised in that each pixel includes a programmable element across the capacitance switchable between a low resistance state in which it shorts the capacitance and a high resistance state.
By supplying a programmable element in each pixel element, the programmable elements can be programmed in accordance with a static image so that with appropriate voltages supplied to the data and select lines the image displayed is in accordance with the state of the programmable elements in the various pixels. By programming all of the elements to be in a high resistance state, the matrix display according to the invention can be operated in a conventional manner to display dynamic pictures.
Accordingly, the matrix display can conveniently be used to display either static or dynamic pictures as required. When static pictures are displayed there is no need to supply repetitive data signals and there is a considerable reduction in power consumption.
The programmable element is preferably a metal semiconductor metal structure, and in particularly preferred embodiments an amorphous silicon carbide structure of thickness, for example, between 50 nm and 100 nm may be used as the semiconductor layer. Alternative semiconductor metal semiconductor structures such as amorphous silicon nitride, amorphous silicon, polysilicon or multi-layer structures may also be used.
The capacitance storing charge may be a separate capacitor, or alternatively the programmable element may provide the capacitance for storing charge without the need for an additional component.
Embodiments of the invention provide a light emitting diode type matrix display. The light emitting diode may be connected between first and second voltage rails in series with a pixel transistor having a control terminal connected to the select transistor, and the capacitance and programmable element may be connected between the control terminal of the pixel transistor and the first voltage rail. In this way, the voltage across the capacitance controls whether the pixel transistor conducts to allow current to flow through the light emitting diode so that the pixel is bright or whether the pixel transistor is switched off so that the pixel is dark.
Embodiments of the invention may provide a photo-diode connected across the capacitance for receiving light emitted from the light emitting diode to provide negative feedback.
Other embodiments of the invention relate to a liquid crystal matrix display.
The invention also relates to a method of operation of a matrix display as described above, including operating the matrix display in a static mode by: programming the programmable elements of a set of the pixels into a low resistance state, and applying a predetermined voltage to all the data lines to display an image corresponding to the set of pixels programmed into the low resistance state.
The method may further comprise operating the matrix display in a dynamic mode by programming all the programmable elements to be in the high resistance state, and displaying image data by sequentially storing charge on the capacitances of selected pixels so that an image is displayed corresponding to those pixels selected. It is also possible for part of a picture to be static while the remainder is dynamic, for example a static logo in one corner.